01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish and set aside.
02 - Bring a large pot of salted water to a boil. Add the pasta and cook until al dente, approximately 1–2 minutes less than the package directions. Drain thoroughly and set aside.
03 - While the pasta is cooking, steam or blanch the chopped broccoli florets for 2–3 minutes until bright green and slightly tender, then drain.
04 - Melt butter in a large saucepan over medium heat. Add minced garlic and sauté until fragrant, about 1 minute.
05 - Pour heavy cream into the saucepan and bring to a gentle simmer. Whisk in 1 cup grated Parmesan until the sauce is thickened and smooth. Season with salt, black pepper, and a pinch of nutmeg if desired.
06 - In a large mixing bowl, combine cooked pasta, chicken, broccoli, and Alfredo sauce. Mix thoroughly until all components are evenly coated.
07 - Transfer the mixture to the prepared baking dish. Evenly sprinkle shredded mozzarella and 1/4 cup grated Parmesan over the top.
08 - Bake in the preheated oven for 20–25 minutes until the casserole is bubbly and the cheese has turned golden.
09 - Let the bake stand for 5 minutes before serving. Garnish with chopped parsley if desired.